It is well known that diisocyanates which are liquid at room temperature (i.e., about 25.degree. C.) have numerous advantages over solid diisocyanates because they are easier to mix and work with. However, diisocyanates which are liquid at room temperature and which are used on a large technical scale, such as toluene diisocyanate or hexamethylene diisocyanate, are as a rule physiologically harmful due to their high vapor pressure and, therefore, can only be used if certain safety precautions are taken. For this reason, various attempts have been made, either to start with diisocyanates that are normally liquid at room temperature and to reduce their physiological effects by certain procedures or to start with diisocyanates that are solid at room temperature and to convert these into liquid form. In both cases, however, one usually obtains either isocyanates of higher valency, i.e., tri- or polyisocyanates or higher molecular weight diisocyanates or a combination of these effects.
The most important diisocyanates which are solid at room temperature and which are readily available on a large commercial scale are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof which melt at 39.degree. C. and 34.5.degree. C., respectively. Attempts have already been made to liquefy both the 4,4'-diphenylmethane diisocyanate and a mixture of the 4,4'-diphenylmethane diisocyanate and a small amount of the 2,4'-isomer. Thus, for example, in U.S. Pat. No. 3,644,457, l mol of a diphenylmethane diisocyanate is reacted with from about 0.1 to about 0.3 mols of poly-1,2-propylene ether glycol. While the products made according to this patent have met with commercial success, they still suffer from a serious drawback. Specifically, it has been found that these adducts generally will crystallize anywhere from 5.degree. C. to as high as 25.degree. C. In fact, when supplied in commercial quantities, these adducts are generally transported in heated trucks. Additionally, in order to thaw the materials, it is generally necessary to heat them to somewhere in excess of 50.degree. to 60.degree. C. While in warmer climates, there may not be any problem, in colder areas where the product may be stored in tanks over a period of time, this tendency to crystallize can become a very serious problem. Similar attempts to form liquid diphenylmethane diisocyanates have been described, for example, in U.S. Pat. Nos. 3,384,653; 3,449,256 and 3,394,164. The attempts to liquefy in these instances were based on the addition of, in one case, a trihydrocarbyl phosphate and, in the other case, small amounts of phosphoric acid. In any event, the storage stability of both of these types of products is again quite good around room temperature, but as the temperature decreases, both types of materials tend to crystallize.
There are numerous other patent references relating to the production of liquid diphenylmethane diisocyanates. Some of the approaches taken include the reaction of the isocyanate with N,N-di(2-hydroxypropyl) aniline in the presence of phosphoric acid (U.S. Pat. No. 3,394,165); the introduction of carbodiimide groups into the isocyanate (U.S. Pat. Nos. 4,177,205; 3,641,093; 3,640,966; 4,014,935; 3,152,162; 4,088,665; 4,072,712; 4,143,063 and 4,154,752); and the reaction of the isocyanate with (i) polyoxyethylene glycols (U.S. Pat. Nos. 4,115,429 and 4,055,548); (ii) propylene glycols (U.S. Pat. Nos. 4,118,411 and 3,892,691); (iii) N,N'-disubstituted thioreaus (U.S. Pat. Nos. 3,674,828); and (iv) low molecular weight polyether polyols (U.S. Pat. No. 4,102,833). Other approaches include the addition of an organosilicone (U.S. Pat. No. 3,585,230); the addition of a dibenzoate (U.S. Pat. No. 3,701,796); and the combination of partial carbodiimidization with a reaction product of the isocyanate and a diol (U.S. Pat. No. 4,031,026). It is apparent that the art is continuing its search for commercially acceptable liquid diphenylmethane diisocyanates.
One of the currently used methods of producing a liquid diphenylmethane diisocyanate involves the reaction of difunctional polypropylene glycol with diphenylmethane diisocyanate (see, e.g., U.S. Pat. No. 3,644,457). The resultant liquid product consists of unreacted diphenylmethane diisocyanate and various adducts of the glycol with the isocyanate. The adducts are generally those produced from two mols of isocyanate and one mol of glycol, three mols of isocyanate and two mols of glycol, four mols of isocyanate and three mols of glycol, and higher adducts. The freezing point depression of the isocyanate caused by the presence of the adducts can be varied by adjusting the number of adducts. In general, as the amount of adduct molecules increases, the freezing point decreases and the viscosity increases. Necessarily, practical constraints are placed on the amount of glycol that can be added such that the product is liquid while, at the same time, the product has a low enough viscosity for ease of processing.
The adducts formed as defined above must be significantly soluble in the isocyanate. Short chain diols, such as ethylene glycol, 1,4-butane diol and 1,6-hexane diol, form adducts with diphenylmethane diisocyanate which are insoluble in the diisocyanate and precipitate from solution at 45.degree. C. (i.e., above the melting point of the isocyanate itself). Adducts of these types of diols can be made soluble by using a diol having alkyl-substitution and by using a diol having oxygen linkages in the backbone. It appears that both ether linkages and alkyl substitution are necessary. Thus, 1,3- and 2,3-butane diol and diethylene glycol form solids or gels when reacted with excess diphenylmethane diisocyanate, while di- and tripropylene glycol form liquids with such isocyanate.
It is also known that the introduction of urea linkages in a polyurethane polymer has an advantageous effect including increasing stiffness, improving high temperature properties and improving tensile strength. However, it has not been thought that a urea group could be incorporated in a diphenylmethane diisocyanate while at the same time producing a liquid product.
It is, therefore, an object of this invention to provide improved liquid organic diisocyanates which are liquid and stable at temperatures even lower than room temperature. A further object of this invention is to provide organic diisocyanates which remain liquid even on storage at low temperatures. It is yet a further object of the present invention to produce a liquid isocyanate which contains urea linkages.